Stimulating the spinal cord for the purpose of controlling pain was first implemented based upon the gate control theory of pain. Simply stated, the gate control theory is based on the premise that activation of large-diameter afferent nerve fibers causes an inhibition of activity in small-diameter nerve fibers. Since small-diameter fibers are involved in the perception of pain their inhibition leads to a consequent inhibition in the perception of pain. As an alternative to the gate control theory some researchers propose that, rather than a physiological gating mechanism, the activation of action potentials in the dorsal columns of the spinal cord leads to a functional blocking of signals in the collaterals of the dorsal columns which, when activated, add to the perception of pain. Under either theory the objectives and principles of spinal cord stimulation for pain control remain the same.
Pain inhibition by activation of large-diameter fibers is directly related to the area or segment of the spinal cord being stimulated. For example, to inhibit pain occurring in the foot, stimulation must activate the large-diameter fibers carrying sensory information from the foot to the spinal cord and higher brain centers. The objective of spinal cord stimulation is to induce sensory paresthesia in such a way that it broadly covers the area in which the patient feels pain. Thus, the proper location of the stimulation electrode is critical to successful pain control.
It is well known that various areas of the body are associated with the dorsal roots of nerve fibers at various spinal segments. Since the dorsal columns receive additional nerve fibers at each spinal segment, the relative position of the nerve fibers from a particular area in the periphery change from the lower spinal segments to the cervical segments. For effective pain control the electrode must be placed adjacent to the spinal column rostral to the dorsal root associated with the painful area.
It is equally well known that stimulation of the dorsal columns at different points medial to lateral will evoke paresthesia perceived as coming from different locations of the body. Additionally, the sensory fibers in the dorsal columns travel to the medulla on the same side of the cord as the peripheral area which they represent. Pain on the right side of the body is treated by placing the electrode to the right of the midline. Pain on the left side of the body is treated by placing the electrode to the left side of the midline. Bilateral pain is treated by placing the electrode on the midline or by placing electrodes on both sides of the midline. Thus, successful pain control through spinal cord stimulation depends on proper positioning of the stimulating electrode both in the longitudinal or rostral-caudal direction and in the lateral to medial direction.
Typically, implantable spinal cord stimulating leads contain multiple electrodes. Two basic styles are available. One style is the percutaneously inserted lead which is introduced through a Touhy needle. The implanting physician places the electrode in an appropriate location using fluoroscopic visualization. The procedure is done under a local anesthetic. Proper electrode placement is tested using a trial stimulation screening technique to assure that paresthesia is perceived in the affected area. An example of this type of lead is disclosed in U.S. Pat. No. 4,379,462 issued to Borkan. That lead has at least three in-line electrodes equally spaced along the distal end of the lead and is designed to be inserted so that the electrodes lie in-line along the spinal cord. Although different pairs of electrodes may be selected so that the area of stimulation may be moved longitudinally along the midline of the spinal cord, there is no provision for stimulating laterally to either or both sides of the midline unless the lead is inserted to one side of midline. In that case once the lead is placed there is no ability to stimulate other than unilaterally on the side of the midline to which the lead is placed. Should the patient later develop the need for bilateral stimulation the physician generally has three options. The physician may reposition the existing lead, implant an additional lead, or remove and replace the existing lead. Percutaneously inserted leads of this type provide focused stimulation patterns and are generally suited for unilateral pain problems. If the pain is bilateral it is often necessary to implant two leads, one on each side of the midline of the spinal cord. The leads may be connected to one pulse generator or to two pulse generators. The use of two leads can cause problems since it is difficult to maintain the relative positions of the leads with respect to one another, both in the longitudinal and lateral directions. Migration of one or both of the leads may result in a loss of paresthesia at the affected location.
The second basic spinal cord stimulation lead type are those surgically implanted through a laminotomy. An example of this type of lead is the RESUME (reg. trademark) lead manufactured by Medtronic, Inc. of Minneapolis, Minn., the assignee of the present invention. This lead has four in-line electrodes located on an elongate paddle at the distal end of the lead. The lead is normally implanted so that the electrodes lie over the midline of the spinal cord. Because leads of this type are surgically implanted, the size of the electrodes may be made larger than those of the percutaneously implanted leads. Various electrode combinations may be selected so that the area of stimulation may be moved along the midline of the spinal cord. The lead provides a broader stimulation pattern more suitable for midline and bilateral pain problems than the percutaneously inserted lead. Since it is surgically implanted it can be sutured to prevent dislodgement and reduce lead migration. In situations where longitudinal placement of the lead over the midline of the spinal cord has not been effective to produce bilateral paresthesia this lead has been placed at an angle with respect to the midline. Once the lead has been inserted at an angle across the spinal cord it is possible, by selection of appropriate electrodes, to stimulate unilaterally on either side of the spinal cord or bilaterally across the spinal cord. However, it is no longer possible to change the stimulation pattern longitudinally along the midline. Additionally, although unilateral stimulation on either side may be provided, the stimulation areas are asymmetric or at different dorsal root levels with respect to the dorsal column. Further, since it is very difficult to maintain the precise angled placement of the lead, any migration of the lead may result in a loss of paresthesia at the affected location.
Another example of a surgically implanted lead is disclosed in U.S. Pat. No. 3,724,467 issued to Avery et al. In one embodiment the lead consists of a flat body portion at the distal end of the lead with electrodes grouped on either side of the longitudinal axis of the lead. The lead is meant to be implanted within the dura and is used for use bilateral stimulation of the spinal cord. In another embodiment the electrodes are mounted on one side of the longitudinal axis of the lead and are meant to provide stimulation to only one side of the spinal cord. In neither embodiment is there any provision for altering the stimulation pattern other than by changing the location of the lead. Thus, once this lead has been implanted there is no way to change the area of stimulation to correct for any loss of paresthesia.
In addition to the problem of lead migration as noted above it is often desirable to effect a change in the area of stimulation in order to vary the effects of paresthesia as the needs of the patient change. The problem of lead migration and the ability to effectively vary the area of stimulation both longitudinally and laterally are areas in which prior art leads have been unable to adequately address. For example, percutaneously inserted leads are difficult to anchor and have a tendency to become dislodged. Even if the initial placement is accurate, lead migration can occur which can adversely affect paresthesia. Additionally, the area in which the patient is experiencing pain can move. Percutaneous leads provide only limited means to change the area of stimulation if the lead migrates or if the needs of the patient change. This is a significant problem with respect to percutaneous leads since the electrodes must be made small enough to fit through a Touhy needle. The area of stimulation is consequently small and even a slight movement of the lead, especially laterally, can adversely affect paresthesia.
Surgically implanted leads are less affected by the problem of lead migration because the electrodes are usually larger and the lead may be stabilized by sutures. However, in instances where lead migration does occur prior art leads have allowed for changes in stimulation only longitudinally along the axis of the lead. There is no mechanism to effect a change of stimulation laterally. The same limitations apply when the needs of the patient change and it becomes desirable to alter the paresthesia. Thus, it would be desirable to have a lead for use in spinal cord stimulation having electrodes which could be selected for stimulation placed such that the area of stimulation could be varied longitudinally, medial to lateral on either side or any combination thereof.